Deck 9: Molecular Biology

Question 1

Figure 9.10 You isolate a cell strain in which the joining together of Okazaki fragments is impaired and suspect that a mutation has occurred in an enzyme found at the replication fork. Which enzyme is most likely to be mutated?

Answer 1

The double helix structure of DNA (Deoxyribonucleic Acid) was proposed by Francis Crick and James Watson who said that DNA is composed of two strands which are twisted around each other and form a helix that is right-handed.
Replication of DNA is called the semi conservative method of replication as each strand of the double helix structure behaves as the template from which new DNA is made. Therefore, each new double strand or double helix consists of one new strand known as the daughter strand and one strand from the parent known as the parental strand.
The process of replication of DNA occurs in three main steps as follows:
i. DNA gets unwound at the origin of replication, forming a replication fork.
ii. DNA polymerase adds new bases to each of the parental strands which are complementary to one another. As DNA polymerase can only synthesize DNA in the 5' to 3' direction, therefore one strand is the leading strand where continuous DNA synthesis occurs while the other is the lagging strand where short pieces known as Okazaki fragments are formed in a discontinuous fashion.
iii. The Okazaki fragments use primers made of RNA to continue synthesis. The RNA primers are replaced with DNA nucleotides and gaps sealed by DNA ligase.
In a situation where the joining together of Okazaki fragments is impaired, a mutation in the enzyme DNA ligase is the most likely as this enzyme is responsible for the joining together of the Okazaki fragments in the lagging strand of DNA.

Question 2

Which of the following does cytosine pair with?

A) guanine
B) thymine
C) adenine
D) a pyrimidine

Answer 2

The double helix structure of DNA (Deoxyribonucleic Acid) was proposed by Francis Crick and James Watson who said that DNA is composed of two strands which are twisted around each other and form a helix that is right-handed.
Adenine and guanine are purines which are double-ringed structures whereas cytosine and thymine are pyrimidines which are smaller and single-ringed structures. Base pairing occurs between purines and pyrimidines. Adenine pairs with thymine whereas guanine pairs with cytosine.
Cytosine being a pyrimidine cannot pair with thymine and a pyrimidine. Also, adenine and cytosine do not pair with each other due to Chargaff's Rule.
Hence, the options b., c. and d. are incorrect.
According to Chargaff's rule, in a molecule of DNA there is as much thymine as adenine and as much cytosine as guanine. Therefore, cytosine will pair with only guanine, according to this rule.
Hence, the correct answer is option

Question 3

Prokaryotes contain a ________chromosome, and eukaryotes contain ________ chromosomes.

A) single-stranded circular; single-stranded linear
B) single-stranded linear; single-stranded circular
C) double-stranded circular; doublestranded linear
D) double-stranded linear; double-stranded circular

Answer 3

Chromosomes are organized and packaged structures which contain the DNA (Deoxyribonucleic Acid) of living organisms.
Prokaryotes have plasmids to store the DNA as they do not have a well-defined nucleus. These chromosomes are circular in shape and are double stranded.
Therefore, prokaryotes do not have single-stranded circular or linear and double stranded linear chromosomes.
Hence, the options a., b. and d. are incorrect.
Prokaryotes have their chromosome in a double-stranded and circular form which is found in the nucleoid which is an area of the cytoplasm. The chromosomes of eukaryotes on the other hand are double-stranded and linear which are wound around each other to form the DNA. The DNA of eukaryotes is wrapped around histones in order to form nucleosomes.
Hence, the correct answer is option

Question 4

DNA replicates by which of the following models?

A) conservative
B) semiconservative
C) dispersive
D) none of the above

Question 5

The initial mechanism for repairing nucleotide errors in DNA is ________.

A) mismatch repair
B) DNA polymerase proofreading
C) nucleotide excision repair
D) thymine dimers

Question 6

A promoter is ________.

A) a specific sequence of DNA nucleotides
B) a specific sequence of RNA nucleotides
C) a protein that binds to DNA
D) an enzyme that synthesizes RNA

Question 7

Portions of eukaryotic mRNA sequence that are removed during RNA processing are ________.

A) exons
B) caps
C) poly-A tails
D) introns

Question 8

The RNA components of ribosomes are synthesized in the ________.

A) cytoplasm
B) nucleus
C) nucleolus
D) endoplasmic reticulum

Question 9

How long would the peptide be that is translated from this MRNA sequence: 5'- AUGGGCUACCGA-3'?

A) 0
B) 2
C) 3
D) 4

Question 10

Control of gene expression in eukaryotic cells occurs at which level(s)?

A) only the transcriptional level
B) epigenetic and transcriptional levels
C) epigenetic, transcriptional, and translational levels
D) epigenetic, transcriptional, posttranscriptional, translational, and posttranslational levels

Question 11

Post-translational control refers to:

A) regulation of gene expression after transcription
B) regulation of gene expression after translation
C) control of epigenetic activation
D) period between transcription and translation

Question 12

Describe the organization of the eukaryotic chromosome

Question 13

Describe the structure and complementary base pairing of DNA.

Question 14

How do the linear chromosomes in eukaryotes ensure that its ends are replicated completely?

Question 15

Transcribe and translate the following DNA sequence (nontemplate strand): 5'- ATGGCCGGTTATTAAGCA-3'.

Question 16

Describe how controlling gene expression will alter the overall protein levels in the cell.